Group 4 Complexes of a tert-Butylphosphine-Bridged Biphenolate Ligand

The coordination chemistry of group 4 complexes supported by the tridentate, dianionic biphenolate phosphine ligand that carries a phosphorus-bound tert-butyl group, 2,2′-tert-butylphosphino-bis(4,6-di-tert-butylphenolate) ([tBu-OPO]2−), is described. Metathetical reactions of {[tBu-OPO]Li2(DME)}2 with 2 or 1 equiv of TiCl4(THF)2 selectively produce [tBu-OPO]TiCl2(THF) (1a) and Ti[tBu-OPO]2 (2a), respectively. Protonolysis of Ti(OiPr)4 with 2 or 1 equiv of H2[tBu-OPO] cleanly generates 2a and [tBu-OPO]Ti(OiPr)2 (3a), respectively. Complex 1a can alternatively be prepared from comproportionation of 2a with 1 equiv of TiCl4(THF)2. Treatment of 1a with 2 equiv of NaOtBu affords [tBu-OPO]Ti(OtBu)2 (4a). In contrast, reactions of {[tBu-OPO]Li2(DME)}2 with ZrCl4(THF)2 or HfCl4(THF)2, regardless of stoichiometry of the starting materials employed, selectively give bis-ligated M[tBu-OPO]2 [M = Zr (2b), Hf (2c)]. Comproportionation of 2b,c with MCl4(THF)2 (M = Zr, Hf) leads to the formation of [tBu-OPO]MCl2(THF) [M = Zr (1b), Hf (1c)], which, upon being treated with 2 equiv of NaOtBu, generates [tBu-OPO]M(OtBu)2(THF) (4b,c). These synthetic results are markedly different from those obtained from analogous reactions employing a biphenolate phosphine ligand bearing a phosphorus-bound phenyl group ([Ph-OPO]2−), highlighting a profound phosphorus substituent effect on complex conformation. The alkoxide complexes 3a and 4a−c are all active initiators for catalytic ring-opening polymerization of ε-caprolactone. To assess the potential phosphorus substituent effect on catalysis, [Ph-OPO]Ti(OiPr)2 (5a) was prepared, and its reactivity was examined. Interestingly, polymers prepared from 3a are characterized by low polydispersities with molecular weights that are linearly dependent on the monomer-to-initiator ratio, thus featuring a living system. The polydispersitiy indexes of polymers prepared from 5a, however, are relatively larger, indicative of the significance of the phosphorus-bound tert-butyl group in 3a in view of discouraging the undesirable transesterification.